Webclient: revise topicmap view model (#443).

The default topicmap view model is now renderer agnostic.
It is not coupled to a HTML5 Canvas 2D Context anymore.

BREAKING CHANGES

DefaultTopicmapRenderer.Model
    - class is renamed to DefaultTopicmapViewmodel
    - add_topic() returns the created topic view model (formerly void)
    - add_association() returns the created association view model (formerly void)

Canvas Topic (the topic view model):
    - "truncated_label" is a new property

Close ticket 443.